1. Field of the Invention
The present invention relates to a process for producing a biaxially oriented nylon single-layered or multilayered film. It can be used as food packaging, industrial material and the like.
2. Description of the Related Arts
A nylon film which has been simultaneously biaxially oriented by a tubular film process has superior features in that its mechanical and optical characteristics, such as a mechanical strength and transparency, are good.
A multilayered film of nylon 6 layer/polyvinylidene chloride layer (i.e., PVDC) is employed as a substrate which acts as a barrier to water. However, there has been a problem in that incinerating the multilayered film produces harmful chlorine gas, which causes acid rain which, in turn, causes environmental destruction. Thus, various multilayered nylon films which do not contain polyvinylidene chloride have been proposed as a substrate which do not cause environmental problems and also have a superior characteristic as a barrier to oxygen. Since, e.g., a multilayered film comprising a nylon layer, adhesive resin layer and polyolefin layer has superior thermal shrinkage characteristics as well as is a superior barrier, a demand that it be used for a fresh meat packaging film or the like has been expected. Hitherto, a process for stably producing such biaxially oriented nylon multilayered film has been required. In addition, a multilayered film of nylon 6 layer/ethylene-vinyl acetate copolymeric saponification product (i.e., EVOH)/nylon 6 layer has been proposed and a process for stably producing this biaxially oriented multilayered film has been required.
In accordance with a prior-art tubular film process for producing a biaxially oriented nylon film, since the thickness accuracy of the resulting film is generally low, it has tended to have a poor winding appearance and produce defects in secondary processing of the film, e.g., a printing, lamination or bag-making, which has restricted food packaging and industrial uses of the film. This is because a thickness unevenness of a 2-6% order will appear even when an extrusion die adjusts the thickness accuracy of the film in producing a rollstock film to be oriented. In addition, the thickness unevenness will at least double during the progress of film drawing in accordance with the prior-art tubular film process and since the prior-art tubular film process could fail to stabilize a bubble under a film drawing operation, the bubble could rock and occasionally rupture.
Hitherto, in order to secure molding stability in a biaxial draw operation of a polyamide resin film, a process in which preheating is applied to the polyamide resin film before a biaxial drawing operation thereof (Unexamined Japanese Patent Application Publication No. 57-41924), a process in which a draw magnification is regulated (Examined Japanese Patent Application Publication No. 49-47269), a process in which a draw temperature is regulated (Examined Japanese Patent Application Publication No. 53-15914) and the like have been proposed. However, none of these processes could accurately regulate desirable process conditions.
In particular, since orientation stress in a film such as a multilayered film of nylon 6 layer/EVOH layer/nylon 6 layer is too high to continuously mold it in accordance with a process in which preheating is applied before biaxial drawing (Unexamined Japanese Patent Application Publication No. 57-41924), it is unpreferable.
Examined Japanese Patent Application Publication No. 49-47269 propose a process for producing a biaxially molecularly oriented poly-.epsilon.-capramide resin film comprising the steps of: quenching and solidifying a tubular film made of a molten and extruded poly-.epsilon.-capramide resin to obtain an essentially amorphous and hydrogen bond-free poly-.epsilon.-capramide resin tubular film in a state of less than 2% water content; and performing tube drawing at a temperature of 45.degree. to 70.degree. C. inclusive before film drawing so that respective machine direction draw magnification and transverse direction draw magnification of the tube draw are in the ranges of 2.0 to 4.0.
Examined Japanese Patent Application Publication No. 53-1 5914 proposed a process for biaxially drawing a tubular polyamide film comprising the steps of: heating a nonoriented tubular polyamide film at a temperature of 50.degree. to 90.degree. C. inclusive; then holding an atmospheric temperature between a draw start point and draw completion point of 180.degree. to 250.degree. C. inclusive to fix the draw start point; and performing a simultaneous biaxial draw by gas pressure at a machine direction draw magnification of 2.5 to 3.7 and at a transverse direction draw magnification of 3.0 to 4.0 while restricting the final machine and transverse direction draw magnification differences to within the ranges of 0.2 to 0.6. However, such processes with the draw magnifications or draw temperature being controlled could not accurately regulate process conditions for producing a good film.
In addition, since nylon 6-66 is a resin having superior shrinkage characteristics and mechanical strength, processes for stably producing single-layered and multilayered nylon films made of nylon 6-66 for packaging film substrate, e.g., for animal meat or cheese has been demanded. In the meanwhile, when a polyvinylidene chloride resin, which has good shrinkage characteristics but low mechanical strength, is employed for packaging film substrate, a corresponding packaging film can be ruptured, especially in a cold chain.
In addition, since nylon 66 is a resin having good heat resistance and mechanical strength, a process for stably producing a monolayered or multilayered nylon film made of nylon 66 for retorted food and high-retorted food packaging film substrates has been demanded.